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The Weather Network
Oct. 22, 2020 | Thursday
Editorials and Opinions
Arch-i-text: It all works together
Mortar Loss hidden by Pebbledash Parging. (Supplied/Brian Marshall)

Was your old house built before 1900? If so, it’s almost certain that the basement foundation was constructed with lime mortar; a completely different critter than today’s Portland mortars.

The master mason working on our Centre Street restoration project described it this way: “These old buildings are never completely still. When they move, it’s the lime mortar that does the work. Think of the lime as a living thing. When the masonry shifts, tiny cracks open up in the mortar. Then with the first hard rain, the lime is activated and heals the cracks.”

But, he continued, “just like anything that lives, the lime has a lifespan which is eventually used up. You know it’s time for repair when you see that those little cracks are just not going away after a few solid rainstorms.”

Historically, the masons constructed buildings in a fashion that encompassed the nature of materials they worked with. Foundations were set on undisturbed soil and usually made of stone laid in a manner that was stable even as it shifted.

The brick, softer than its current-day counterpart, was set in bonds that maintained integrity while moving with the foundation. The buildings were made to "breathe," moving water vapour through the walls as part of a natural renewing process.

And all of the components were bound together with a mortar that respected the materials, the movement of the building and "self-healed" to last through decades.

It was a marvellous system, provided that a little maintenance was performed with compatible materials when necessary. Unfortunately, the “innovations” of the 20th century combined with the idea that “newer is always better” have, and are, leading to significant damage to these old buildings.

As an example, let’s consider something that has been commonly done over the last 100 years: parging rubblestone foundations with the "new" Portland product. It would last longer and look just as good.

The problem was that the original lime parging was not simply for looks. It was meant to be a sacrificial layer to protect and extend the life of the lime mortar. When the parging needed replacement, one repaired any deteriorated mortar as well.

While Portland parging does last significantly longer, it is by its nature static, doesn’t work with the original system, and the lime mortar hidden behind inevitably loses its structural integrity. 

As a result, what holds up your old house might not be working like it used to.

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